Mr Crispy posted...
And I never read one piece but aren't devil fruit powers caused by well, devils? At least I remember hearing the explanation that people could only have one devil fruit power because if they ate more than one the two devils would fight and kill the host.
No, that's just a rumor that the people of the world made up as an explanation.
Right now it's a mystery as to what devil fruits are and how they grant powers.
Chances are it's a perfectly physical thing, as the sea can negate their powers. Material has been made that does the same thing, as does the use of haki, a form of energy that is inherent to all people, if not living things.